Evaluating two undef values for string equality yields a true result. undef is temporarily promoted to "" (the empty string) in string context. If warnings are enabled, warnings are emitted.

c:\@Work\Perl\monks>perl -le
"use strict;
 use warnings;
 ;;
 use Data::Dump qw(dd);
 ;;
 my ($value, $alice, $bob);
 ;;
 $value = $alice eq $bob;
 dd 'result of comparison of undefined values', $value;
"
Use of uninitialized value in string eq at -e line 1.
Use of uninitialized value in string eq at -e line 1.
("result of comparison of undefined values", 1)
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My response is 'undef'